Work will continue on an investigation by small-angle x-ray scattering of the structure of E. coli ribosomes in solution. Radius of gyration measurements will be made on the 50S ribosome in H2O and D2O. The measurements will be made on samples prepared by Engleman and Moore of Yale University for comparison with their measurements by neutron scattering on the same samples. Experiments will be done to elucidate the nature of the 50S to 45S transition on addition of 0.5 M NH4Cl to 50S ribosomal subunits. The reversibility of the transition will be investigated in terms of the sedimentation coefficient, the radius of gyration and the diffusion coefficient.